Sensor with a catalytically active protective layer for determining the oxygen content in gases, and process for manufacturing such a sensor

ABSTRACT

A sensor for determining the oxygen content in exhaust gases, particularly for lambda probes, is presented with catalytically active substances in the porous protective layer, which cover the electrode or electrodes exposed to the measuring gas, with discretely distributed platinum and homogenously distributed rhodium being used as catalytically active substances. The sensor in accordance with this invention guarantees excellent catalytic conversion of the exhaust gases and thus a control position of the sensor close to lambda=1.

BACKGROUND OF THE INVENTION

The invention relates to an electrochemical sensor for determining theoxygen content in gases, particularly in exhaust gases of internalcombustion engines. A sensor of this general type is already known(DE-OS 37 37 215), and has a solid electrolytic body, at least oneelectrode on the side of the solid electrolytic body exposed to themeasuring gas, as well as a protective layer with catalytically activesubstances on the electrode or electrodes exposed to the measuring gas,with the catalytically active substance being platinum or a platinumalloy. The incorporation of catalytically active substances into theporous protective layer which covers the electrode exposed to themeasuring gas, is necessary in order to accelerate the setting of thethermodynamic equilibrium of the exhaust gas, and to achieve a controlposition as close as possible to λ=1. However, it has been shown that atlow exhaust gas temperatures, platinum is not capable of catalysing theequilibrium setting to a satisfactory degree. On the other hand, rhodiumdemonstrates an excellent catalytic effect at low temperatures, inparticular with regard to the reduction of nitric oxides. In thepublication SAE Paper no. 880557 "Multi-Layered Zirconia Oxygen Sensorwith Modified Rhodium Catalyst Electrode", a lamina-shaped oxygen probeis proposed which has a rhodium layer on a platinum electrode, with aninsulating intermediate layer being provided, where appropriate, betweenthe platinum and rhodium layers. Through this layer-type construction,the equilibrium setting in the exhaust gas, and thus also the probecharacteristics, are improved. The platinum electrodes and the rhodiumcatalyst are applied in layers one on top of the other, by means ofscreen printing. Thus at least two operating steps are necessary, andthe consumption of precious metals is relatively high. If the platinumor rhodium layers are applied directly one on top 8 the other, without aseparating insulating layer, then in addition an alloy formationresults, whereby a portion of the rhodium loses its catalytic effect.The raw material, rhodium is, however, more than five times moreexpensive than platinum, and the amount used should therefore be kept aslow as possible.

SUMMARY OF THE INVENTION

In contrast, the sensor in accordance with this invention, and theprocess of its manufacture, have the advantage that very small amountsof material, in particular minimal amounts of rhodium, are employed inan optimum manner to improve the sensor control position, in particularat low temperatures. In particular, the present invention provides foran electrochemical sensor for determining the oxygen content in gases,particularly in exhaust gases of internal combustion engines. The sensorcomprises a solid electrolyte composed of stabilized zirconium dioxideand/or other oxides which conduct oxygen ions, having at least oneelectrode on the side of the solid electrolyte exposed to the gas to bemeasured, and also having a porous ceramic protective layer on theelectrode or electrodes exposed to the gas to be measured, containingactive catalytic metals. One of the catalytic metals which is active athigh temperatures is a discretely distributed component. This componentis preferably platinum. The other active catalytic metal, which isactive at low temperatures is a homogeneously distributed component.This component is preferably rhodium.

The sensor in accordance with this invention can, moreover, bemanufactured in a manner which is simple from the point of view ofproduction engineering.

For one thing, an adequate catalytic action at increased exhaust gastemperature is achieved through discrete distribution of the activecomponents preferred at increased temperature, in particular platinum,into the porous protective layer, since at high temperatures, gasdiffusion and catalytic reactions occur at an accelerated rate. Foranother thing, through the homogenous distribution of the componentswhich are active at low temperatures, in particular rhodium, excellentcatalytic effectiveness is achieved at low temperatures. Due to thedistribution of the two catalytic metals in accordance with thisinvention, it is further guaranteed that the number of direct contactpoints between the two catalytic metals, and thus the undesirable alloyformation, is low.

In order to achieve the distribution of the catalytic metals inaccordance with this invention, it has been shown to be particularlyadvantageous to add platinum, as a powder, and rhodium as an aqueous ororganic solution of rhodium compounds, to the raw materials for formingthe porous protective layer. For homogenous precipitation of extremelypure rhodium, an aqueous nitrate solution and an organic resinatesolution have been shown to be particularly suitable.

The catalytic metals or the substances forming the catalytic metals canbe mixed with the remaining raw materials forming the porous protectivelayer, e.g. aluminum oxides, magnesium spinel and/or zirconium dioxide,as well as, if appropriate, a bonding agent, a softener and/or a vehiclesubstance and applied to the previously manufactured electrode orelectrodes in one operation e.g., by spraying, immersion or printing,followed by rescinerting. However, it is also possible to initially addjust the platinum powder to the remaining raw materials and to completethe porous protective layer, and in one further operation, to impregnatethe same with rhodium solutions and subsequently to release the rhodiumthrough thermic treatment, preferably in a reducing or an inertatmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The Figure shows a section of an electrochemical sensor according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the figures, the solid electrolyte 10 carries the electrode11 which is exposed to the gas to be measured, with this electrode 11 inturn being covered by the porous ceramic protective layer 12. In theprotective layer 12, a component which is catalytically active at hightemperature is distributed discretely, and a component which iscatalytically active at low temperature is distributed homogenously.

The invention is described below on the basis of examples. The porousceramic protective layer of the present invention preferably contains0.5 to 5 percent by weight platinum and 0.05 to 0.5 percent by weightrhodium; and most preferably, contains 1 percent by weight platinum and0.2 percent by weight rhodium. The sensor in accordance with thisinvention guarantees excellent catalytic conversion of the exhaust gasesand thus a control position of the sensor close to λ=1.

EXAMPLE 1

A mixture of 65 parts by weight of ceramic material, consisting of 60parts by weight of -Al₂ O₃ powder (>99% Al₂ O₃ ; specific surface 10 m²/g) and 40 parts by weight of ZrO₂ powder (>99% ZrO₂ ; specific surface1.5 m² /g), 2 parts by weight of polyvinyl alcohol bonding agent, 1 partby weight of platinum powder (>99% platinum; specific surface 15 m² /g),0.4 parts by weight of rhodium nitrate (>99% Rh (NO₃)₂), the remainderwater, is ground for 6 hours in a Vibratom mill.

The dispersion thus obtained is applied through spraying, in a layerdensity of about 200 μm, onto the portion of a solid electrolyte bodyequipped with an electrode layer. The sensor is then resintered forabout 3 hours at a temperature in the region of 1400° C.

In this manner, a porous protective layer, with excellent catalyticactivity at both high and low temperatures, is obtained over theelectrode or electrodes exposed to the gas to be measured.

EXAMPLE 2

The following are ground together in a centrifugal ball mill:

60 parts by weight of an Al₂ O₃ /ZrO₂ mixed oxide powder, obtainedby-co-precipitation, with 40 per cent by weight ZrO₂ with a specificsurface of about 10 m² /g,

5 parts by weight polyvinyl butyral as a bonding agent,

2.5 parts by weight dibutylphthalate as softener,

1 per cent by weight platinum powder (>99% platinum; specific surface 15m² /g),

1.5 per cent by weight rhodium resinate (17.8% rhodium), and

30 per cent by weight butylcarbitol as the vehicle.

The mixture is ground for 2 hours and subsequently applied by screenprinting to the electrode or electrodes of an oxygen sensor of afamiliar type. After drying, it is resintered for 2 hours at atemperature in the region of 1400° C.

EXAMPLE 3

By stirring for 15 minutes, a suspension is produced which consists of:

65 parts by weight of an Al₂ O₃ /ZrO₂ mixed oxide powder, as describedin example 2,

1 part by weight of platinum powder (>99% platinum; specific surface 15m² /g),

0.2 per cent by weight rhodium resinate and

3.5 parts by weight of a vehicle system, consisting of 4.0 parts byweight terpineol, 60 parts by weight benzyl alcohol and 6 parts byweight ethyl cellulose.

The suspension is applied to the portion of a solid electrolyte sensorequipped with electrodes, by immersion or spraying, dried for half anhour at a temperature in the legion of 100° C., and subsequentlyresintered for 3 hours at a temperature in the region of 1400° C.

A porous protective layer with excellent catalytic characteristics athigh and low temperatures is obtained.

EXAMPLE 4

The starting point of this process a suspension as described in example3, but without the addition of the rhodium resinate. The suspension isapplied to the electrodes, dried and resintered, as described in example3. The finished protective layer is subsequently impregnated with anaqueous 10% rhodium nitrate solution, and baked for 1 hour at 900° C. ina reducing atmosphere. Again, a porous protective layer with excellentcatalytic characteristics is obtained.

We claim:
 1. An electrochemical sensor for determining the oxygencontent in gas, comprising:(a) a solid electrolyte composed of an oxygenconducting material; (b) at least one electrode on the side of the solidelectrolyte exposed to the gas to be measured; (c) a porous ceramicprotective layer on said at least one electrode, said protective layercontaining at least one catalytic metal which is active at hightemperatures and which is discretely distributed therein, and at leastone catalytic metal which is active at low temperatures which ishomogeneously distributed therein, with said at least one catalyticmetal active at high temperatures including platinum and said at leastone catalytic metal active at low temperatures being rhodium.
 2. Thesensor of claim 1 wherein the solid electrolyte is stabilized zirconiumdioxide, oxygen ion conducting oxides or mixtures of these oxides. 3.The sensor of claim 1 wherein the porous ceramic protective layercontains 0.5 to 5 percent by weight platinum.
 4. The sensor of claim 3wherein the porous ceramic protective layer contains 0.05 to 0.5 percentby weight rhodium.
 5. The sensor of claim 1 wherein the porous ceramicprotective layer contains 0.05 to 0.5 percent by weight rhodium.
 6. Thesensor of claim 1 wherein the porous ceramic protective layer contains 1percent by weight platinum and 0.2 percent by weight rhodium.
 7. Aprocess for producing an electrochemical sensor for determining theoxygen content in a gas, comprising the steps of:(a) providing a solidelectrolyte comprising stabilized zirconium dioxide, oxygen ionconducting oxides or mixtures of these oxides; (b) providing at leastone electrode on a side of the solid electrolyte to be exposed to a gasto be measured; (c) forming a ceramic material mixture containingplatinum, as a catalytic metal active at high temperatures, and rhodium,as a catalytic metal active at low temperatures, by mixing and grindingtogether platinum containing powder, ceramic raw materials and a liquidrhodium solution; (d) applying the ceramic material mixture as aprotective layer to said at least one electrode; and (e) drying andsintering the ceramic material mixture of the protective layer whereby aporous ceramic protective layer is formed wherein the platinum isdiscretely distributed and the rhodium is homogeneously distributed. 8.The process of claim 7 wherein the liquid rhodium solution is an aqueousrhodium nitrate or a rhodium resinate solution.
 9. The process of claim7 wherein the ceramic raw material includes aluminum oxide, zirconiumdioxide, magnesium spinel or a mixture of these substances.
 10. Theprocess of claim 7 wherein binders, solvents and softeners areadditionally added to the mixture during said step of forming.
 11. Theprocess of claim 7 wherein said step of applying includes applying theceramic material to the electrode by spraying, dipping or printing. 12.The process of claim 7 wherein the porous ceramic protective layer onthe electrode is at least 150 micrometers thick subsequent to sintering.13. The process of claim 7 wherein the platinum containing powder isplatinum powder.
 14. A process for producing an electrochemical sensorfor determining the oxygen content in a gas, comprising the steps of:(a)providing a solid electrolyte comprising stabilized zirconium dioxide,oxygen ion conducting oxides or mixtures of these oxides; (b) providingat least one electrode on a side of the solid electrolyte to be exposedto a gas to be measured; (c) forming a ceramic material mixture bymixing and grinding together platinum containing powder and ceramic rawmaterials; (d) applying the ceramic material mixture as a protectivelayer on said at least one electrode; (e) drying and sintering theceramic material of the protective layer to form a porous ceramicprotective layer containing platinum as a catalytic metal active at hightemperatures; and (f) thereafter, impregnating the platinum containingporous ceramic protective layer with a liquid rhodium solution anddrying the impregnated layer, whereby a porous ceramic protective layercontaining discretely distributed platinum as a catalytic metal which isactive at high temperatures and homogeneously distributed rhodium as acatalytic metal which is active at low temperatures.
 15. The process ofclaim 14 wherein the liquid rhodium solution is an aqueous rhodiumnitrate or a rhodium resinate solution.
 16. The process of claim 14wherein the ceramic raw material is aluminum oxide, zirconium dioxide,magnesium spinel or a mixture of these substances.
 17. The process ofclaim 14 wherein binders, solvents and softeners are additionally addedto the mixture during said step of forming.
 18. The process of claim 14wherein said step of applying includes applying the ceramic materialmixture to the at least one electrode by spraying, dipping or printing.19. The process of claim 14 wherein the porous ceramic protective layeron the at least one electrode is at least 150 micrometers thicksubsequent to sintering.
 20. The process of claim 14 wherein theplatinum containing powder is platinum powder.